Add an API function to get a debugger stack trace from an error handle.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 74117cc890c7626dfab5cecd7e978bde797cb284..a87f25d347f65317554f63d531186c7851fed213 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -4250,6 +4250,7 @@ DART_EXPORT Dart_Handle Dart_LoadLibrary(Dart_Handle url,
   if (library.IsNull()) {
     library = Library::New(url_str);
     library.Register();
+    library.set_debuggable(true);
   } else if (!library.LoadNotStarted()) {
     // The source for this library has either been loaded or is in the
     // process of loading.  Return an error.